Much work has been done to remove pollutants from emissions from coal fired furnaces. The focus of most of these efforts has been toward the removal of particulates, NOx and SOx from flue gas. Commercially available techniques for reducing nitrogen oxide emissions in furnace flue gases include low-NOx burners, overfire air, selective non-catalytic NOx reduction (SNCR), selective catalytic reduction (SCR), and reburning.
Reburning is a technique whereby a fraction of the total thermal input to the furnace is injected above the primary combustion zone to create a fuel rich zone. Hydrocarbon fuels such as coal, oil, or gas are more effective NOx reducers than non-carbon containing fuels such as hydrogen or non-hydrogen containing fuels such as carbon monoxide. Stoichiometry of about 0.90 (10% excess fuel) in the reburn zone is considered optimum for NOx control. Thus, it is apparent that the amount of reburn fuel required for effective NOx control is directly related to the stoichiometry of the primary combustion zone and, in particular, the amount of excess air therein. Under typical furnace conditions, a reburn fuel input of over 10% of the total fuel input to the furnace is usually sufficient to form a fuel-rich reburn zone. The reburn fuel is injected at high temperatures in order to promote reactions under the overall fuel-rich stoichiometry.
Typical flue gas temperatures at the injection point are above 1700 K (2600° F.). Overfire air is introduced into the flue gases downstream of the fuel-rich reburn zone in order to complete combustion of any unburned hydrocarbons and carbon monoxide (CO) remaining in the flue gases leaving the fuel-rich reburn zone. In addition, it is also known that rapid and complete dispersion of the reburn fuel in the flue gases is beneficial. Thus, the injection of reburn fuel is frequently accompanied by the injection of a carrier fluid, such as recirculated flue gases, for the purpose of promoting mixing. To the extent that the recirculated flue gas contains oxygen, the amount of reburn fuel must be increased.
U.S. Pat. No. 5,443,805 teaches injection of an additive such as ammonia with a small amount of hydrocarbon, preferably methane or natural gas, into flue gases at a temperature in the range of about 1228 K to 1422 K (1750° F. to 2100° F.), and preferably 1355 K to 1338 K (1800° F. to 1950° F.), for reducing pollutants such as NOx therein. Hydrocarbon is injected for the purpose of enhancing the NOx reduction efficiency of the ammonia additive in the temperature range of about 978 K to about 1422 K (1300° F. to 2100° F.). There is a similar teaching in U.S. Pat. No. 6,258,336. That patent also teaches that other nitrogenous compounds such as amines, urea, cyanuric acid and mixtures thereof can be injected with a hydrocarbon fuel downstream of the primary combustion zone.
While the art has focused primarily on the removal of NOx and SOx from flue gas, there are also concerns about emissions of mercury and other elemental metals such as chromium, arsenic and lead from combustion devices. Mercury (Hg), the eightieth element, is an important pollutant. As a vapor it is a poison of the nervous system. It is the dire consequences of chronic mercury poisoning which gave birth to the term “Mad as a hatter.” Hatters that used mercury to block the hats were exposed to toxic levels of mercury vapor. The tremors, shakes, stutters, and stammers common to mercury poisoning were endemic in the trade. Neither were astronomers, who frequently used telescopes that were floated on mercury, immune from this disease. It was at times fatal and has the characteristic of being cumulative over years of exposure, as the body's nervous system has difficulty in purging this element. Most industrial uses of mercury today are carefully controlled. The biggest sources of environmental mercury are coal combustion and the combustion of municipal solid waste. Coal and especially municipal solid waste compositions may also result in emissions containing chromium, arsenic and lead.
Mercury vapor is a poison. At the levels common in the atmosphere the concentrations are usually safe. However, the mercury accumulates in lakes where it is further accumulated in fish. These fish, with organic mercury molecules in them, can be hazardous to individuals who eat them. Some states request that people eat fish from some lakes no more frequently that once a week. Often it is stated that pregnant women and small children should eat no such fish.
Several states and the United States Environmental Protection Agency will soon limit the emissions of mercury and possibly other elemental metals from combustion devices. A method for controlling emissions of mercury and other metals is needed. Some control is possible by using particulate collection devices. However, only very expensive baghouses (fabric filters) are efficient enough to reduce the mercury to levels that may be required and still it is possible for the elemental mercury vapor to escape as a gaseous vapor molecule.
Activated carbon and other fine particulates are used to absorb mercury. Special treatment of the activated carbon has been tested. Collection by the use of activated carbon is very expensive. So, it is seen that a new method of removing mercury from flue gas is needed.